1. Field of the Invention
The present invention relates to an apparatus and a method for pulling a single crystal and, more particularly, to an apparatus and a method for pulling a single crystal of silicon or the like by a pulling method such as the Czochralski method (hereinafter, referred to as the CZ method).
2. Description of the Relevant Art
At present, a majority of silicon single crystals used for manufacturing a substrate for forming a circuit component of a large scale integrated circuit (LSI) and the like have been pulled by the CZ method. As a method for excluding dislocations from a pulled single crystal, a method called the Dash necking method has been used in general, wherein a diameter of a crystal is narrowed down to about several millimeters so as to exclude dislocations. Recently, as a pulled single crystal has a larger diameter, weight of the single crystal has been increasing. Accordingly, a load imposed on a slender neck might exceed the tensile strength of silicon, so that a risk of a fall of the crystal during pulling a single crystal has increased.
In order to cope with the above risk, two methods have been developed: one method (See Japanese Kokai No. 1999-189488) wherein by preheating a seed crystal using a movable auxiliary heating device, induction of dislocations caused by a thermal shock of contact of the seed crystal with a melt is inhibited, so that a single crystal having no dislocation is pulled without forming a neck; and the other method (See Japanese Kokai No. 1999-189488) wherein by heating a neck using an auxiliary heating device during a dislocation-excluding activity according to the Dash necking method, a temperature distribution of the neck is controlled so as to reduce thermal stress affecting the neck, so that even with the neck having a larger diameter than usual, a single crystal having no dislocation can be pulled.
The applicant of the present invention previously proposed an apparatus for pulling a single crystal having an auxiliary heating device comprising a heating section which can be located so as to surround a seed crystal in a position near and above a melt charged in a crucible, and a transfer mechanism for withdrawing the heating section from a passing area of a single crystal (Japanese Kokai No. 2000-137986).
FIG. 9 is a sectional view schematically showing a conventional apparatus for pulling a single crystal having an auxiliary heating device. FIGS. 10(a) and 10(b) are a perspective view and a plan view, respectively, schematically showing a form of a heating section constituting the auxiliary heating device in the conventional apparatus for pulling a single crystal.
Reference numeral 21 in FIG. 9 represents a crucible, comprising a bottomed cylindrical quartz crucible 21a and a bottomed cylindrical graphite crucible 21b fitted on an outer side of the quartz crucible 21a. The crucible 21 is supported by a support shaft 28 which rotates in a direction shown by arrow A in FIG. 9 at a prescribed speed. A main heater 22 of a resistance heating type and a heat insulating mold 27 around the main heater 22 are concentrically arranged around the crucible 21. The crucible 21 is charged with a melt 23 of a material for forming a crystal which is melted by the main heater 22. On a central axis of the crucible 21, a pulling axis 24 made of a pulling rod or wire is suspended, and at a tip thereof, a seed crystal 35 is held by a holder 24a. 
Reference numeral 25 in FIG. 9 represents a straightening vane. A main body 25a of the straightening vane 25 has a shape of a sidewall of an inverted truncated cone, is located so as to surround a pulled single crystal 36, and is arranged so that a lower end portion thereof can be positioned near and above a surface of the melt 23 charged in the crucible 21.
Reference numeral 26 in FIG. 9 represents an auxiliary heating device. A heating section 26a of the auxiliary heating device 26, as shown in FIGS. 10(a) and 10(b), surrounds half or more of a horizontal perimeter of the seed crystal 35 and has an opening 26b for withdrawing from the seed crystal 35, and is located so as to be able to surround the seed crystal 35 in a position near and above the melt 23. To the heating section 26a, an electrode 26c for supplying power to the heating section 26a and for descending or ascending the heating section 26a is connected. And a transfer mechanism (not shown), for withdrawing the heating section 26a from a passing area of a single crystal 36 during formation of a main body 36c after forming a neck 36a, is installed. The auxiliary heating device 26 comprises this heating section 26a, electrode 26c and transfer mechanism. Here, a heating area of the heating section 26a is shown with hatch lines in FIG. 10(b). These members, except the transfer mechanism, are assembled in a chamber 29 of a water cooled type wherein pressure can be controlled.
A method for pulling a single crystal 36 using the above apparatus for pulling a single crystal is described below by reference to FIGS. 11(a)–11(e). FIGS. 11(a)–11(e) are enlarged partial front views schematically showing a seed crystal and the vicinity thereof in part of steps in pulling a single crystal.
Although it is not shown in FIGS. 11(a)–11(e), the pressure in chamber 29 is reduced and an inert gas is introduced into the chamber 29 so as to provide an inert gas atmosphere under reduced pressure within the chamber 29. And then, material for forming a crystal is melted using the main heater 22 and is maintained for a period of time so as to sufficiently release gas contained in the melt 23.
While the pulling axis 24 is rotated on the same axis in a reverse direction of the support shaft 28 at a prescribed speed, the seed crystal 35 held by the holder 24a is caused to descend and is preheated (FIG. 11(a)). The seed crystal 35 is caused to descend so that a front portion 35a thereof is dipped into the melt 23 (FIG. 11(b)).
Then, while heating an interface between the seed crystal 35 and the melt 23 using the auxiliary heating device 26, the seed crystal 35 is caused to further descend to be immersed in the melt 23 (FIG. 11(c)).
The seed crystal 35 is pulled at a prescribed speed so as to form neck 36a, having almost the same diameter as the seed crystal 35, at the lower portion thereof. At this time, by heating an interface between the neck 36a and the melt 23 using the heating section 26a of the auxiliary heating device 26, thermal stress originating from a temperature distribution of the neck 36a is reduced, so that dislocations are excluded from the neck 36a (a dislocation-excluding step, FIG. 11(d)).
By actuating the transfer mechanism (not shown), the heating section 26a is withdrawn from the neck 36a, and by slowing a pulling speed of the pulling axis 24 (hereinafter, simply referred to as the pulling speed), the neck 36a is made to grow to have a prescribed diameter, leading to formation of a shoulder 36b (a shoulder formation step). By pulling the pulling axis 24 at a fixed rate, a main body 36c having a uniform diameter and a prescribed length is formed (a main body formation step, FIG. 11(e)).
Although it is not shown in FIGS. 11(a)–11(e), in order to prevent induction of high density dislocation to the single crystal 36 by a steep change in temperature at an end, the diameter thereof is gradually decreased and a temperature of the entire single crystal 36 is gradually lowered, leading to formation of an end-cone. Then, the single crystal 36 is separated from the melt 23. Cooling the single crystal 36 is completion of the pulling of the single crystal 36.
In the above conventional apparatus for pulling a single crystal, by heating the interface between the seed crystal 35 and the melt 23 using the heating section 26a of the auxiliary heating device 26, thermal shock from contact with the melt 23 is reduced, so that a number of induced dislocations in contact therewith can be decreased. And by heating the neck 36a, a temperature gradient in a radial direction of the neck 36a is made smaller. Therefore, thermal stress is decreased and a capability of excluding dislocations in the neck 36a is enhanced, so that a single crystal having no dislocation can be pulled.
However, the heating section 26a comprises a heater of a resistance heating type being U-shaped in a plan view with a heating area thereof set to be 50–70% of the horizontal perimeter of the seed crystal 35, so that most of heat from the heating section 26a is upwardly radiated from a clearance between the heating section 26 and the seed crystal 35. Since a total amount of heating applied to the seed crystal 35 or the neck 36a is reduced, it is difficult to make a radial temperature gradient thereof smaller. As a result, there is a possibility that thermal stress is caused, leading to induction of dislocations.
And a side surface of the crystal is heated using the heating section 26a during growing the neck 36a, which is contrary to a physical phenomenon that a crystal is solidified by removing heat, being a fundamental principle of crystal growth. Therefore, this process is formed on a delicate heat balance. While growth of the neck 36a requires skill of an operator, the pulling speed during the growth thereof cannot be made so fast as a general pulling speed (2–4 mm/min) in a conventional necking step. As a result, the neck 36a cannot be grown at a higher speed than a speed at which induced dislocations in the neck 36a propagate, so that it is hard to exclude dislocations from the neck 36a. 